This application proposes a continuation of a program of research to investigate sensory and perceptual processes in spoken communication. The research aims to develop an empirical description and a principled account of the perceptual organization of speech. This perceptual function establishes the coherence of the speech signal despite its acoustic diversity and complexity, By resolving auditory activity into vocal sources, this fundamental function determines the constituents of the perceptual analysis of the linguistic properties of speech. Four projects comprising twelve experiments are proposed for the coming five years. The research includes a set of studies with adult listeners to determine the relation between auditory and phonetic perceptual organization;a detailed psychophysical evaluation of the perceptual stability of phonetic distinctions;a series of tests of the contributions of naturalness to the resolution of phonetic attributes;and, an appraisal of the perceptual reliance on time-varying acoustic structure that elicits the perception of a talker's identity as well as the spoken message. Together the experiments will elaborate fundamental processes of detecting and maintaining perceptual coherence. This research aims to understand how the listener copes with apparently limitless variation in the superficial acoustic constituents of speech. In emphasizing the perceptual utility of coarse-grain spectrotemporal properties, these studies can begin to explain the robustness of speech perception when auditory qualities are altered or diminished by disease, and offers an opportunity to identify therapeutic accommodations for failure or loss of perceptual resources.